Methods for bleaching aqueous pulps and compositions for use in these methods

ABSTRACT

The present invention relates to compositions for use in an aqueous pulp bleaching process, comprising one or more alkaline earth metal oxides and/or one or more alkaline earth metal hydroxides; one or more alkaline earth metal salts; and optionally, one or more pitch control additives. Methods and kits for the formation of said compositions, are also part of the invention, as is the use of said compositions, methods and kits.

FIELD OF THE INVENTION

The present invention relates to compositions for use in the bleaching of pulps. In particular, the compositions are suitable for use as additives for adjustment of pH and/or control of unwanted substances within a pulp slurry during a pulp bleaching step. Also part of the present invention is the use of said compositions, for example the use of said compositions in a pulp preparation or a papermaking process, as well as bleaching methods for pulps using said compositions. Finally, kits of ingredients for forming the compositions according to the invention, or for use in a method according to the invention, are also part of the present invention.

BACKGROUND OF THE INVENTION

Traditional bleaching processes of pulps commonly employ alkaline hydrogen peroxide as a bleaching agent. To that effect, hydrogen peroxide and a water soluble alkaline compound, such as sodium hydroxide and/or water glass are introduced into the pulp in order to improve whiteness and/or brightness of a wood pulp, a recycled paper pulp or any other chemical or mechanical pulp, such as a pulp for papermaking processes. The use of chelating agents like EDTA to remove some of the metal ions from the pulp prior to adding peroxide allows the peroxide to be used more efficiently. Magnesium salts and sodium silicate may also be added to improve bleaching with alkaline peroxide. Magnesium salts typically are also used to protect cellulose fibres from too much degradation and therefore strength loss during delignification and bleaching process steps.

More recently, alkaline earth metal hydroxides and/or alkaline earth metal oxides have been employed to replace sodium hydroxide and water glass, and to reduce the need for chelating agents and sodium silicate (see for example as disclosed in Li et al., Pulp & Paper Canada 106:6 (2005), pages T125 to T129 “Using magnesium hydroxide (Mg(OH)₂) as the alkali source in peroxide bleaching at Irving paper”). WO 2014/195478 A1 discloses the use of pitch control additives in combination with Mg(OH)₂ and/or MgO in paper bleaching pulps, in order to avoid the formation of magnesium based fatty acid salts and resins in paper pulps.

Alkaline earth metal hydroxides and alkaline earth metal oxides are only sparingly soluble in aqueous solutions, which may lead to a reduction in bleaching efficiency and reduced efficiency in fibre protection. In addition, it is known to use MgSO₄ in paper bleaching pulps as a radical scavenger and a slurry stabilising agent. MgSO₄ is expensive and difficult to store, due to its hygroscopic properties.

Finally, maintaining high pulp viscosity is a consideration in the processing of pulps, including in pulp bleaching. Pulp viscosity is a measurement that indicates the strength of the fiber. The set-up of pulp processing plants, such as for example pulp bleaching plants, is notably designed to maintain the pulp viscosity while achieving desired bleaching result. Variation of pulp viscosity can therefore be problematic.

The state of the art therefore presents several problems.

SHORT DESCRIPTION OF THE INVENTION

The present invention is defined in the appended claims.

In particular, the present invention is embodied by a composition for use in an aqueous pulp bleaching process, the composition comprising (i) one or more alkaline earth metal oxides and/or one or more alkaline earth metal hydroxides; (ii) one or more alkaline earth metal salts; and (iii) optionally, one or more pitch control additives. It was found that in the presence of an alkaline earth metal salt, such as an alkaline earth metal sulphate, the alkalinity of the described chemical preparation was reduced, leading to improved solubility of the alkaline earth metal oxide and/or alkaline earth metal hydroxide in this preparation that is then dosed into the pulp, while at the same time the beneficial radical scavenging and slurry stabilising effects of the alkaline earth metal salt, such as an alkaline earth metal sulphate, were obtained. For example, the alkaline earth metal may be selected from magnesium, calcium, or a mixture of magnesium and calcium.

According to one embodiment, the alkaline earth metal salt is an alkaline earth metal sulphate, such as for example calcium sulphate or magnesium sulphate.

According to one embodiment, the pitch control additive is selected from talc, bentonite, zeolite, diatomaceous earth, cationic mica, hydrophobic carbonates, resin-decomposing enzymes, cationic polymers designed to capture and immobilise resins or resinous components on fibre surfaces, aluminium sulphate, polyaluminium chloride, and dispersing agents specifically designed for the dispersion of resins and resinous components, and mixtures thereof. It was found that these pitch control agents lead to a reduction in fatty acid salts and resins in paper pulps. According to one embodiment, the pitch reduction agent is talc, such as for example micronized talc, bimodal talc, cationic talc, or mixtures thereof. According to one embodiment, the pitch control additive is bentonite, such as an activated bentonite. In certain embodiments, the bentonite is activated by the alkaline earth metal oxide and/or alkaline earth metal hydroxide and/or the alkaline earth metal salt. Bentonite may bind heavy metals in the paper bleaching composition and reduce their interference with the bleaching process, for example by degrading the peroxide responsible for the bleaching process.

According to the present invention, the composition may be a dry particulate composition, or the composition may be an aqueous slurry, or the composition may be present as a constituent of an aqueous slurry comprising other components, such as for example, the composition may be present as a constituent of an aqueous paper bleaching pulp. According to the present invention, a particulate composition according to the invention may be mixed with water in order to prepare an aqueous slurry, which is also a part of the present invention.

According to the present invention, the composition may be an aqueous slurry having a solids content of up to 75 wt.-%, such as for example 10 to 75 wt.-%, on the basis of the total weight of the slurry.

According to the present invention, the weight ratio of (i) the alkaline earth metal oxide and/or the alkaline earth metal hydroxide on the one hand and (ii) the alkaline earth metal salt on the other hand is from 1:10 to 100:1. It was found that the advantageous effects according to the present invention were best obtained if the components are present in such proportions. For example, the said weight ratio may be in the range of 1:5 to 50:1, such as for example in the range of 1:2 to 25:1, or from 1:1 to 10:1, such as for example about 2:1, or about 70:30 or about 80:20, or about 90:10.

According to one embodiment, the composition may further comprise one or more of the following additives: dispersants; fillers; surfactants; bleaching agents; chelating agents; and pH-buffering agents.

According to a further aspect of the present invention, there is provided a kit of ingredients, comprising a first container comprising an alkaline earth metal oxide and/or an alkaline earth metal hydroxide, and either one or both of a second and a third container, the second container comprising an aqueous mineral acid solution and the third container comprising sulphur dioxide or carbon dioxide. It was found that the compositions according to the present invention could be easily obtained using the said kit. In one embodiment, the aqueous mineral acid is aqueous sulphuric acid.

Also part of the present invention are methods for producing the compositions of the present invention, including a step of mixing an alkaline earth metal oxide and/or an alkaline earth metal hydroxide with an acid such as a mineral acid, such as sulphuric acid, or sulphur dioxide, or carbon dioxide, or an alkaline earth metal salt, such as for example an alkaline earth metal sulfate or an alkaline earth metal carbonate, optionally in the presence of one or more pitch control agents. The alkaline earth metal may be selected, for example, from magnesium, calcium and mixtures thereof.

According to one aspect of the present invention, the said method may be carried out in situ within an aqueous pulp bleaching composition. It was found that the alkaline earth metal oxide and/or alkaline earth metal hydroxide efficiently reacted in situ with an acid such as a mineral acid, such as sulphuric acid, and/or sulphur dioxide or carbon dioxide to form a mixture of alkaline earth metal oxide and/or alkaline earth metal hydroxide with alkaline earth metal salt, such as an alkaline earth metal sulphate, in situ. For example, the method according to the present invention may be performed by addition of a mineral acid, such as for example sulphuric acid, or carbon dioxide, or sulphur dioxide to a paper bleaching pulp comprising the alkaline earth metal oxide and/or the alkaline earth metal hydroxide. For example, the method according to the present invention may be performed by addition of the alkaline earth metal oxide and/or alkaline earth metal hydroxide with an acidic pulp composition (e.g. wherein the only acid present in the acidic pulp composition is acid that is naturally present in the pulp composition (i.e. derived from the wood) or no external acid is added).

According to one aspect of the invention, the method may include a step of mixing an alkaline earth metal hydroxide with a mineral acid, wherein the base-acid normality, i.e. the molar ratio of hydroxide ions in the alkaline earth metal hydroxide and the hydrogen cations in the mineral acid is between 10:1 and 1:5, such as for example about 10:1, or about 5:1, or about 4:1, or about 3:1, or about 2:1, or about 1.5:1, or about 1:1, or about 1:1.5, or about 1:2, or about 1:5. Corresponding acid/base ratios shall be employed in the case of alkaline earth metal oxide, and carbon dioxide or sulphur dioxide, for example.

According to one aspect of the present invention, the method may be performed by admixing the ingredients of the kit according to one aspect of the present invention into a paper bleaching pulp. It was found that the said kit was easily transported, stored and applied for use in the method of the invention.

Also part of the present invention is the use of the composition, the method or the kit as described according to the present invention in a pulp bleaching process. It was found that the pulp bleaching process could be rendered more simple, efficient and less expensive by the use of the present invention, while at the same time reducing the need for expensive raw materials or complex procedures.

According to some embodiments, the composition, the method or the kit according to the present invention may be used in a process to reduce pitch in a pulp and/or in a process to protect the pulp from depolymerisation and/or degradation of pulp (e.g., resulting in the formation of stickies). In this context, methods for reduction of depolymerisation and/or degradation of a pulp and/or for protecting pulp from the formation of degradation products (e.g., stickies) are also part of the present invention. The depolymerisation or degradation occurs due to attack of the bleaching chemicals on the cellulose molecules causing reduction of polymer chain length and consequently lower strength properties of the pulp. Very often this degradation is measured as fibre viscosity. Lower chain length by depolymerisation or degradation leads to lower viscosity values and hence lower strength properties which reduced the value of the produced pulp.

According to some further embodiments of the present invention, the composition or the product of the method or one or more of the components of the kit according to embodiments of the present invention are added to the pulp to be bleached, prior to an addition of any other alkaline bleaching agents, such as water glass, NaOH, sodium silicate, and/or hydrogen peroxide.

It is understood that the following detailed description and references to the figures concern exemplary embodiments of the present invention and shall not be limiting the scope of the claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention according to the appended claims provides improvements for paper pulping processes, including bleaching, such as oxygen bleaching, peroxide-oxygen bleaching, oxygen-peroxide bleaching or oxygen delignification, for example. The improvements can be obtained by the products and kits as defined in the appended claims, as well as by their application according to the methods and uses as defined in the appended claims.

It has been found that the combination of alkaline earth metals hydroxides and/or alkaline earth metal oxides, together with certain alkaline earth metal salts and optionally pitch control agents provides an array of advantages over the state of the art.

One advantage is the reduction of the formation of harmful deposits in the pulp. This is achieved by the use of, on the one hand, the combination of alkaline earth metal oxide and/or alkaline earth metal hydroxide with the alkaline earth metal salt, and on the other hand by the presence of certain pitch control agents.

Furthermore, it was also found that the presence of certain alkaline earth metal salts leads to a reduction of radicals formed in the bleaching process. The presence of radicals, while on the one hand desirable for the bleaching, may lead to the formation of unwanted side products, e.g., from depolymerisation and/or degradation of pulp such as pitch, or “stickies”, which may lead to clotting of the slurry, and damage to the downstream paper treatment installations.

Furthermore, the presence of certain alkaline earth metal salts, such as alkaline earth metal sulphate, leads to a stabilising of the pulp viscosity and improvement of the pulp strength properties. This is important for the usability of the pulp in paper and board production and other pulp end uses. According to some aspects of the present invention, the viscosity stability is obtained by the use of the compositions or kit of the invention, and leads to a maintenance of viscosity or to a reduction of the decrease in viscosity. For example, according to the present invention, the use of the compositions or ingredients of the kit leads to a change in viscosity which is no more than 10%, such as for example no more than 5%, or no more than 4%, or no more than 3%, or no more than 2%, or no more than 1%, or no more than 0.5%, or which essentially leads to no measurable change in viscosity.

It is thought that this “protection” of the viscosity is caused by the presence of free alkaline earth metal ions, such as magnesium ions, in a slurry. The formation of alkaline earth metal salts from the alkaline earth metal hydroxide or alkaline earth metal oxide and an acid in a paper making pulp leads to the formation of free alkaline earth metals ions, such as magnesium ions and the stabilisation of the paper pulp viscosity.

In addition, the use of alkaline earth metal salts in the present invention allows to adapt the pH-value of the pulp bleaching preparation. Due to the presence of alkaline earth metal oxide and/or alkaline earth metal hydroxide, an alkaline slurry is normally obtained, with an elevated pH of 10 or more, or 11 or more. According to the present invention, the use of alkaline earth metal salt leads to a control of the alkalinity in the pulp bleaching preparation, and it is possible to reduce the pH to a value of between 7 and 10.5, such as for example about 8.5. It is advantageous to work in pH region below 10 or the near neutral region of pH value, in order to protect the equipment from alkali degeneration and evidently for safety reasons. In addition, the lower pH leads to better bleaching performance and/or reduction in pulp depolymerisation and/or degradation. Accordingly, the loading of alkaline earth metal hydroxide and/or alkaline earth metal oxide in the papermaking pulp may be reduced according to the present invention.

Finally, according to some embodiments of the present invention, such as the kit or the method according to the present invention, the said alkaline earth metal salt is obtained by the combination of an alkaline earth metal oxide and/or an alkaline earth metal hydroxide with a mineral acid, or with sulphur dioxide or carbon dioxide in situ before or during a bleaching process. Accordingly, for example, a composition comprising magnesium hydroxide and magnesium sulphate may be obtained within a pulp bleaching preparation by introducing magnesium hydroxide and an acid such as sulphuric acid, according to the following reaction:

Mg(OH)₂+H₂SO₄(aq)→MgSO₄+2H₂O

Acids such as aqueous sulphuric acid are readily available and easy to store in large quantities over prolonged time at a papermaking site. On the other hand, alkaline earth metals salts, such as for example magnesium sulphate, are often expensive to procure and store and difficult to handle, due to their low durability or hygroscopic behaviour. The methods and kits according to the present invention therefore lead to the formation of the composition according to the present invention, and they also form part of the present invention, and present additional advantages over the prior art.

The alkaline earth metal hydroxide or alkaline earth metal oxide may be selected from magnesium hydroxide, calcium hydroxide, magnesium oxide and calcium oxide, or mixtures thereof. If an alkaline earth metal oxide is used, this acts in the same way as an alkalinity agent by formation of the corresponding alkaline earth metal hydroxide in aqueous medium. These compounds have a low solubility in water and are therefore released into the pulp only at the rate they are being consumed during the bleaching process. According to the present invention, control of pH is improved as it can be regulated by the addition of alkaline earth metal salt, or acid, in accordance with the embodiments of the invention.

In certain embodiments, the alkaline earth metal hydroxide may be magnesium hydroxide. In some embodiments, the magnesium hydroxide may be crystalline (e.g., brucite). In other embodiments, the magnesium hydroxide may be an amorphous form.

In certain embodiments, magnesium hydroxide, calcium hydroxide, and mixtures thereof may be provided by dolomitic limestone being converted to dolomitic lime, which is then converted to magnesium hydroxide, calcium hydroxide, and mixtures thereof. Such embodiments including magnesium hydroxide and calcium hydroxide may provide the pulp bleaching benefits described herein as well as a reduction in deposits (e.g., calcium oxalate deposits) and/or a higher availability of soluble hydroxide ions. The ratio of magnesium hydroxide to calcium hydroxide in such embodiments may range from 10:1 to 1:10, such as for example from 5:1 to 1:5, such as for example in the range of 1:2 to 2:1.

The alkaline earth metal salt for use in the present invention may for example be an alkaline earth metal sulphate, such as for example magnesium sulphate. Magnesium sulphate is a hygroscopic particulate salt. The use of the alkaline earth metal salt helps to adjust the pH of the slurry, and may also stabilise the slurry. Alternatively, magnesium chloride or magnesium nitrate may be used.

The pitch control agent for use in the composition according to certain embodiments of the present invention may be talc, a natural magnesium silicate with a platy structure. Alternatively, the pitch control agent for use in the composition according to certain embodiments of the present invention may be bentonite, an absorbent aluminium phyllosilicate including montmorillonite. In particular embodiments, the bentonite may be activated by the alkaline earth metal hydroxide or alkaline earth metal oxide (e.g., Mg(OH)₂ or Mg(OH)). In certain embodiments, the bentonite may act as an ion exchanger for heavy metals such as Mn, Cr, Fe, and/or Ni, which provides the benefit of rendering these ions less available to disturb the bleaching process. In other embodiments, the bentonite may stabilize the peroxide used in bleaching and/or reduce the silicate content.

In certain embodiments, the pitch control agent may be a blend of talc and bentonite. For instance, the ratio of talc to bentonite in such embodiments may range from 10:1 to 1:10, such as for example from 5:1 to 1:5, such as for example in the range of 2:1 to 1:2. This blend of the two different pitch control agents shows the benefit of treating a wider variety of detrimental substances in the process as both substances have different adsorption properties. The hydrophobic surface of talc preferentially adsorbs hydrophobic materials while bentonite has more attraction for hydrophilic substances.

According to certain embodiments of the present invention, the compositions for use in bleaching of pulps may be present as an aqueous slurry. Aqueous slurries are commonly used, for example in papermaking, for introducing solids into a pulp. As such, a composition according to certain embodiments of the present invention in the form of an aqueous slurry may be introduced into the pulp in order to support hydrogen peroxide bleaching. By putting certain embodiments of the composition of the present invention in the form of a slurry, the overall weight of the composition is increased. In order to limit the weight increase, for example to maintain efficiency in transportation of the compositions, it is preferred that the aqueous slurries comprising the composition according to certain embodiments of the present invention have a solids content of 10 wt.-% or higher, on the basis of the total weight of the slurry, such as 30 wt.-% or higher, or 30 wt.-% or higher, or 40 wt.-% or higher, or 45 wt.-% or higher, or 50 wt.-% or higher, or even 60 wt.-% or higher, such as up to 70 wt.-%, or 72 wt.-%, or 75 wt.-%. In order to achieve high solids content slurries, it may be necessary to include dispersants in the slurry. The slurries may also comprise further components such as fillers, surfactants, bleaching agents, pH-buffering agents, or other additives.

In certain embodiments, the compositions have the form of dry particulate powders. In order to reduce weight of the composition and to improve transportability, the compositions may be transported as dry powders and slurries be formed only at the site of use of the compositions in a bleaching process. The said dry particulate powders may be obtained, for example, by use of a spray-drying process. Spray-dried particulate compositions according to certain aspects of the present invention may have a low or a very low moisture content, such as for example 5 wt.-% moisture or less, based on the total amount of solids in the spray-dried particulate powder, or 4 wt.-% moisture or less, or 3 wt.-% moisture or less, or even 2 wt.-% moisture or less, such as for example about 1 wt.-% moisture or 0.5 wt.-% moisture. The dry compositions may also comprise further components such as dispersants, fillers, surfactants, bleaching agents, chelating agents, pH-buffering agents, or other additives.

In certain embodiments, the compositions are in the form of pellets. According to one aspect, the pellets may have a moisture content from 0.1 wt.-% to 25 wt.-%, based on the total weight of the composition, such as for example from 2 wt.-% to 20 wt.-% or from 5 wt.-% to 15 wt.-%, such as for example about 15 wt.-%. According to a further aspect, the pellets may comprise from 2 wt.-% to 50 wt.-% pitch control additive and from 98 wt.-% to 50 wt.-% particulate alkaline earth metal oxide or alkaline earth metal hydroxide or mixtures thereof, based on the total solids content of the pellets.

According to certain embodiments of the present invention, the compositions for use in bleaching of pulps may have a weight ratio of the alkaline earth metal oxides and/or alkaline earth metal hydroxides on the one hand, and the alkaline earth metal salt on the other hand in the range of 1:1 to 100:1, such as for example from 2:1 to 50:1, such as for example in the range of 3:1 to 25:1, or from 5:1 to 10:1, such as for example about 70:30 or about 80:20, or about 90:10.

According to certain embodiments of the present invention, the kit of ingredients may include such amounts of ingredients that a weight ratio as described above may be obtained. It is understood by the skilled person in the art that such weight ratio as a result of the reaction of, for example, magnesium hydroxide with sulphuric acid to obtain magnesium sulphate, as described above, may depend on many factors, such as concentration, temperature, relative amounts of ingredients, acidity/alkalinity prior to the reaction, etc. According to certain embodiments of the present invention, the method for obtaining a composition according to the invention may be carried out in such a way that a weight ratio as described above may be obtained.

If a pitch control agent is used in the present invention, the weight ratio of the alkaline earth metal oxides, alkaline earth metal hydroxides and the alkaline earth metal salt on the one hand, and the pitch control agent on the other hand may be ranging from 1:9 to 9:1, such as for example between 3:7 and 7:3, such as for example ranging from 2:3 to 3:2, for example at a weight ratio of 1:2, or 1:1, or 2:1.

According to certain embodiments of the present invention, the compositions for use in bleaching of pulps may comprise the one or more particulate alkaline earth metal oxides or alkaline earth metal hydroxides in an amount of 10 wt.-% to 90 wt.-% of the total solids content of the composition, such as for example in an amount of 30 wt.-% to 70 wt.-% of the total solids content of the composition, such as for example in an amount of 40 wt.-% to 60 wt.-% of the total solids content of the composition, such as for example in an amount of 50 wt.-% of the total solids content of the composition.

According to certain embodiments of the present invention, the compositions for use in bleaching of pulps may comprise the alkaline metal salt in an amount of 0.1 wt.-% to 50 wt.-% of the total solids content of the composition, such as for example in an amount of 1 wt.-% to 30 wt.-% of the total solids content of the composition, such as for example in an amount of 5 wt.-% to 15 wt.-% of the total solids content of the composition, such as for example in an amount of 10 wt.-% of the total solids content of the composition.

According to certain embodiments of the present invention, the compositions for use in bleaching of pulps may comprise one or more pitch control additives in an amount of 1 wt.-% to 90 wt.-% of the total solids content of the composition, such as for example in an amount of 10 wt.-% to 70 wt.-% of the total solids content of the composition, such as for example in an amount of 20 wt.-% to 60 wt.-% of the total solids content of the composition, such as for example in an amount of 50 wt.-% of the total solids content of the composition.

Another aspect of the present invention is the use of the compositions or kits of ingredients according to certain embodiments of the present invention in the bleaching process of pulps, such as in the bleaching process of pulps for papermaking. For example, the compositions according to certain embodiments of the present invention may be used in combination with other additives such as hydrogen peroxide, complexing agents, water glass, or others, in a bleaching process of pulp in a paper making process.

The compositions according to the present invention may fully or partially replace known alkaline compositions, such as NaOH. For example, NaOH may be fully omitted as the alkaline agent in a H₂O₂-bleaching process, or the amount of NaOH may be reduced by simultaneous addition of a composition according to the present invention.

According to certain embodiments of the present invention, it is not required that the one or more alkaline earth metal oxides or alkaline earth metal hydroxides or mixtures thereof on the one hand, and the one or more alkaline earth metal salts on the other hand, be mixed together prior to being introduced into a pulp to be bleached. An additional aspect of the present invention is the use of a composition obtained by separately introducing one or more (e.g. all) of the components of a composition into a papermaking pulp, or by separately introducing one or more (e.g. all) of the components of a kit of ingredients into a papermaking pulp, thereby forming the composition within a pulp during the bleaching of said pulp.

In the bleaching process, the composition or the kit of ingredients according to certain embodiments of the present invention may be introduced into the pulp in such an amount that the amount of the one or more particulate alkaline earth metal oxides or alkaline earth metal hydroxides or mixtures thereof in the pulp is from 0.1 wt.-% to 8 wt.-% of the total solids content of the pulp during the bleaching, such as from 0.2 wt.-% to 4 wt.-% of the total solids content of the pulp during the bleaching, such as from 0.5 wt.-% to 2.5 wt.-% of the total solids content of the pulp, such as for example 1 wt.-% or 2 wt.-% of the total solids content of the pulp, and that the amount of the one or more alkaline earth metal salts in the pulp is from 0.01 wt.-% to 5 wt.-% of the total solids content of the pulp during the bleaching, such as from 0.02 wt.-% to 1 wt.-% of the total solids content of the pulp during the bleaching, such as from 0.05 wt.-% to 0.5 wt.-% of the total solids content of the pulp, such as for example about 0.1 wt.-% or about 0.25 wt.-% of the total solids content of the pulp.

According to certain embodiments of the present invention, the composition or the ingredients of a kit of ingredients for use in bleaching of a papermaking pulp may be included in said pulp for example during the bleaching stage, or at the beginning of the bleaching stage, or prior to the bleaching stage. It also may be included during or prior to the delignification stage. For example, the composition for use in bleaching of a papermaking pulp may be included in said pulp as early as the grinding stage during which the pulp components are ground, or at any time between the grinding stage and the bleaching stage. The various components of the composition according to certain embodiments of the present invention may also be added to the papermaking pulp at separate stages, such as for example during the grinding stage and or just prior initiation of the bleaching step or the delignification step.

For example, according to one embodiment of the present invention, the composition or the ingredients of a kit of ingredients for use in bleaching of a papermaking pulp according to some embodiments of the present invention may be introduced into a pulp to be bleached prior to the introduction of any alkaline bleaching agents, such as NaOH, waterglass, sodium silicate, and/or hydrogen peroxide. In this case, the alkaline earth metal oxide or the alkaline earth metal hydroxide has been activated by the presence of alkaline earth metal salt (such as earth metal sulphate), and/or organic acids of wood origin present in the pulp mixture, and/or the mineral acid prior to the beginning of the bleaching process. For example, according to one embodiment of the invention, the alkaline earth metal hydroxide and/or alkaline earth metal oxide is added to an acidic pulp mixture (e.g. wherein the only acids present are those naturally found in the pulp and/or no external acid is added) prior to the beginning of the bleaching process and/or prior to the addition of any alkaline bleaching agents.

As discussed above, it is thought that this “protection” of the viscosity is caused by the presence of free alkaline earth metal ions, such as magnesium ions, in pulp mixture or slurry. This effect is enhanced when the alkaline earth metal ions are formed prior to the start of bleaching by introduction of any alkaline bleaching agents.

According to a further embodiment of the present invention, the composition of ingredients or the ingredients of a kit of ingredients may be added at an earlier step, such as during an acidic bleaching step, during the addition of chelating agents such as EDTA or chlorine dioxide. This embodiment includes all uses of the composition of ingredients or the ingredients of a kit of ingredients in a bleaching step which occurs at a pH of 7 or below.

In order to avoid confusion, the embodiments in accordance with the following numbered sentence all form part of the present invention:

1. A composition for use in a pulp bleaching process, the composition comprising one or more alkaline earth metal oxides and/or one or more alkaline earth metal hydroxides; one or more alkaline earth metal salts; and optionally, one or more pitch control additives. 2. A composition according to numbered sentence 1, wherein said pitch control additive is selected from talc, bentonite, zeolite, diatomaceous earth, cationic mica, hydrophobic carbonates, resin-decomposing enzymes, cationic polymers designed to capture and immobilise resins or resinous components on fibre surfaces, aluminium sulphate, polyaluminium chloride, and dispersing agents specifically designed for the dispersion of resins and resinous components, and mixtures thereof. 3. A composition according any of the previous numbered sentences, wherein the alkaline earth metal is selected from magnesium, calcium, and mixtures thereof. 4. A composition according to any of the previous numbered senctences, wherein said pitch control additive is selected from micronized talc, bimodal talc, and cationic talc. 5. A composition according to any one of numbered sentences 1 to 3, wherein said pitch control additive is a bentonite, such as for example activated bentonite. 6. A composition according to any of the previous numbered sentences which is an aqueous slurry. 7. A composition according to numbered sentence 6, wherein said aqueous slurry has a total solids content of from 10 to 75 wt.-%, on the basis of the total weight of the slurry. 8. A composition according to any of the previous numbered sentences, wherein the weight ratio of (i) an alkaline earth metal oxide and/or an alkaline earth metal hydroxide and (ii) an alkaline earth metal salt is from 1:2 to 100:1. 9. A composition according to any of the previous numbered sentences, further comprising at least one of the following additives: (a) dispersants; (b) fillers; (c) surfactants; (d) bleaching agents; (e) chelating agents; and (f) pH-buffering agents. 10. A kit of ingredients comprising, an alkaline earth metal oxide and/or an alkaline earth metal hydroxide stored in a first container, and either one or both of a second and third container with an aqueous mineral acid solution stored in the said second container and sulphur dioxide and/or carbon dioxide stored in the said third container. 11. A method for forming a composition according to any one numbered sentences 1 to 9, comprising a step of mixing an alkaline earth metal oxide and/or an alkaline earth metal hydroxide with a mineral acid, or sulphur dioxide, or carbon dioxide, optionally in the presence of one or more pitch control agents. 12. A method according to numbered sentence 11, when performed within an aqueous pulp bleaching composition. 13. A method according to numbered sentence 12, wherein the step of mixing is performed by addition of sulphuric acid or sulphur dioxide or carbon dioxide to a paper bleaching pulp comprising the alkaline earth metal oxide and/or the alkaline earth metal hydroxide. 14. A method according to any one of numbered sentences 11 to 13, comprising the step of admixing the ingredients of the kit of numbered sentence 10 into a paper bleaching pulp or a delignification pulp. 15. A use of a composition or a method or a kit according to any one of the previous numbered sentences, in a pulp bleaching process or a pulp delignification process. 16. A use according to numbered sentence 15 in a process to reduce pitch in the pulp. 17. A use according to numbered sentence 15 in a process to protect the pulp from depolymerisation and/or degradation. 18. A use according to numbered sentence 15 in or after an acidic bleaching step. 19. A use according to any one of numbered sentences 15 to 18, wherein the said composition or the product of said method or the components of the said kit are added to the pulp to be bleached, prior to an addition of any other alkaline bleaching agents, such as water glass, sodium silicate, NaOH, and/or hydrogen peroxide. 20. A method of reducing pitch in a pulp comprising the use according to any one of numbered sentences 15 to 19.

EXAMPLES

As used herein, pulp viscosity was measured in accordance with TAPPI T 230 om-08. Furthermore, as used herein, the kappa value indicating the residual amount of lignin in the pulp was measured in accordance with ISO-302:2012.

In a bleaching process of a paper pulp using sodium hydroxide as the bleaching agent, the use of magnesium hydroxide as an additive was compared to the use of magnesium hydroxide in combination with mineral acid, as well as in combination with mineral acid and EDTA as a chelating agent.

An unbleached kraft softwood pulp was used as starting material. The pulp was taken from industrial production at a softwood kraft pulp mill. It was washed in the mill and then subjected to a bleaching process without further washing in the laboratory. Commercially available magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used in these trials.

In Comparative Examples 1 and 2, magnesium sulfate and magnesium hydroxide solution respectively were added to the paper making pulp. In Examples 1 and 2, magnesium hydroxide solution and laboratory grade sulfuric acid was added. In Example 3, laboratory grade EDTA (10 g/L) was additionally charged. Sulfuric acid and EDTA were first mixed with the commercial magnesium hydroxide. The additive mixtures are shown in Table I. Then this mixture was mixed to the pulp. In all the Examples, the charge of pulp was 120 g (oven dry). The bleaching was carried out in 2 L a Teflon-lined autoclave, put in a preheated silicone oil bath. The total reaction time was about 2 and a half hours in each case.

TABLE I Comp. Comp. Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex. 3 MgSO₄ (wt.-%) 0.20 0 — — — Mg(OH)₂ (wt.-%) 0 0.15 — — — 1:1 Mg(OH)₂ + H₂SO₄ 0 — 0.15 — — (wt.-%) 2:1 Mg(OH)₂ + H₂SO₄ 0 — 0 0.15 0.15 (wt.-%) EDTA (wt.-%) 0 0 0 0 0.02 The wt.-% indications are with respect to the total dry weight of the pulp

Pulp testing was performed to the usual standards according to German Thünen Institute (Hamburg, Germany). The viscosity was determined according to ISO 5351 using a copper ethylenediamine (cuen) solution. The results are shown in Table II.

TABLE II Comp. Comp. Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex. 3 Brightness (ISO) 38.8 41.8 38.9 36.9 37.8 Pulp viscosity (mL/g) 940 860 920 930 920 Kappa 11.4 9.9 11.8 12.0 11.8 yield 87.43 87.44 87.36 87.34 87.26

It is clear from the obtained data that (i) the viscosity values for the invention Examples are equivalent to those of Comparative Example 1, while the use of magnesium sulfate has been avoided. At the same time, the values for viscosity and kappa value are improved over those of Comparative Example 2. Finally, the values for brightness, kappa value and overall yield were not adversely affected over Comparative Example 1.

Example 3, making use of EDTA in addition to the combination Mg(OH)₂/H₂SO₄, does not substantially modify the expected technical properties of the pulps.

In a separate bleaching process of a paper pulp using sodium hydroxide and hydrogen peroxide as bleaching agents, the effect of using increasing amounts of magnesium hydroxide to replace sodium hydroxide on residual peroxide and brightness was determined.

An unbleached kraft softwood pulp was used as starting material. The pulp was taken from industrial production at a softwood kraft pulp mill. It was washed in the mill and then subjected to a bleaching process without further washing in the laboratory. The water pressed out from the pulp sample used in the laboratory bleaching trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially available magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used in these trials.

Pulp mixtures shown in Table III were prepared by first mixing the additive components together and then adding this mixture to an aqueous pulp. The bleaching was carried out in plastic bags, put in a preheated water bath of 70° C. with the bag opening outside the water to avoid additional water getting into the bag during the bleaching. The total reaction time was about 2 hours in each case.

TABLE III All values Comp. in grams Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Pulp (wet, 100 100 100 100 100 100 16.3% dry pulp content) Water 69.2 69.2 69.2 69.2 69.2 69.1 NaOH 2.2 2.1 2.0 1.8 1.1 0 (7.4%) Mg(OH)₂ 0 0.1 0.2 0.5 1.1 2.2 (5.3%) Mg(SO₄) 0.07 0 0 0 0 0 (12%) H₂O₂ 0.45 0.45 0.44 0.45 0.45 0.45 (30%)

After bleaching for about 2 hours, the residual peroxide was determined by pressing out 1 ml of water from the pulp and using iodometric titration well known by experts in this field and described in literature. ISO brightness of the pulp mixture was also determined from a dried filter cake of the bleached pulp using a Konica Minolta CM3700d spectrocolorimeter following ISO 2469 (R457). The results are shown in Table IV. The chemical oxygen demand (COD) was determined from the water pressed out of the pulp after bleaching using the Hach Lange cuvette test LCK614.

TABLE IV Comp. Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Residual 0 0 0 0 32 45 peroxide (% of original amount added) ISO 83.3 84.1 82.7 83.2 81.3 80.1 Brightness (%) COD 21.4 16.3 18.2 17.3 13.2 13.9 (kg/t)

It was found that magnesium hydroxide can be used to replace sodium hydroxide in the bleaching process when using an acidic pulp mixture. Even at relatively low substitution rates of NaOH by Mg(OH)₂, a significant drop in COD values can be observed. This is advantageous for pulp yield since each kg produced COD is lost pulp yield. It also has a positive effect on the COD in the waste water, which thus provides an improved environmental impact.

In another bleaching process of a paper pulp using sodium hydroxide and hydrogen peroxide as bleaching agents, the effect of adding magnesium hydroxide to the pulp before sodium hydroxide and any other alkaline additives was determined.

An unbleached kraft softwood pulp was used as starting material. The pulp was taken from industrial production at a softwood kraft pulp mill. It was washed in the mill and then subjected to a bleaching process without further washing in the laboratory. The water pressed out from the pulp sample used in the laboratory bleaching trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially available magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used in these trials.

Pulp mixtures shown in Table V were prepared by either mixing the additive components and then adding this mixture to aqueous pulp, or, where the magnesium hydroxide was added to the pulp first, adding the magnesium hydroxide to the aqueous pulp and mixing the pulp mixture by hand for around 1 minute, then mixing the remaining additive components and adding this mixture to the magnesium hydroxide and pulp mixture. The bleaching was carried out in plastic bags, put in a preheated water bath of 70° C. with the bag opening outside the water to avoid additional water getting into the bag during bleaching. The total reaction time was about 80 minutes in each case.

TABLE V Ex. 11 Ex. 13 (mag- (mag- nesium nesium hydroxide hydroxide All values Comp. added added in grams Ex. 4 Ex. 9 Ex. 10 first) Ex. 12 first) Pulp (wet, 100 100 100 100 100 100 16.3% dry pulp content) Water 69.2 69.2 69.2 69.2 69.2 69.2 NaOH 2.2 2.0 1.9 1.9 1.9 1.9 (7.4%) Mg(OH)₂ 0 0.2 0.3 0.3 0.3 0.3 (5.3%) Mg(SO₄) 0.07 0 0 0 0 0 (12%) Bentonite 0 0 0 0 0.04 0.04 (2%) H₂O₂ 0.44 0.44 0.44 0.44 0.44 0.44 (30%)

After bleaching for about 80 minutes, the residual peroxide and ISO brightness of the pulp mixtures were measured as described above. The results are shown in Table VI below.

TABLE VI Ex. 13 (mag- Ex. 11 nesium (magnesium hydroxide Comp. hydroxide added Ex. 4 Ex. 9 Ex. 10 added first) Ex. 12 first) Residual 5 5 4 29 4 35 peroxide (% of original amount added) ISO 83.0 82.5 81.7 82.8 80.8 82.8 Brightness (%)

It was found that the residual peroxide and ISO brightness of the pulp mixtures to which magnesium hydroxide was added first is better than that of the pulp mixture to which magnesium hydroxide was added at the same time as the other additive components.

In another bleaching process of a paper pulp using sodium hydroxide and hydrogen peroxide as bleaching agents, the effect of using a reduced amount of hydrogen peroxide together with magnesium hydroxide was determined.

An unbleached kraft softwood pulp was used as starting material. The pulp was taken from industrial production at a softwood kraft pulp mill. It was washed in the mill and then subjected to a bleaching process without further washing in the laboratory. The water pressed out from the pulp sample used in the laboratory bleaching trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially available magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used in these trials.

Pulp mixtures shown in Table VII were prepared by either mixing the additive components and then adding this mixture to aqueous pulp, or, where the magnesium hydroxide was added to the pulp first, adding the magnesium hydroxide to the aqueous pulp and mixing the pulp mixture by hand for around 1 minute, then mixing the remaining additive components and adding this mixture to the magnesium hydroxide and pulp mixture. The bleaching was carried out in plastic bags, put in a preheated water bath at 70° C. with the bag opening outside the water to avoid additional water getting into the bag during bleaching. The total reaction time was about 80 minutes in each case.

TABLE VII Ex. 15 Ex. 16 (magnesium (magnesium All values in Comp. hydroxide hydroxide grams Ex. 5 Ex. 14 added first) added first) Pulp (wet, 225 225 225 225 16.3% dry pulp content) Water 155.6 155.7 155.8 155.9 NaOH (7.4%) 5.0 4.2 4.2 4.2 Mg(OH)₂ 0 0.8 0.8 0.8 (5.3%) Mg(SO₄) 0.2 0 0 0 (12%) H₂O₂ (30%) 0.98 0.98 0.86 0.73

After bleaching for about 80 minutes, the residual peroxide and ISO brightness of the pulp mixtures were measured as described above. The results are shown in Table VIII below.

TABLE VIII Ex. 15 Ex. 16 (magnesium (magnesium Comp. hydroxide hydroxide Ex. 5 Ex. 14 added first) added first) Residual 3 29 29 23 peroxide (% of original amount added) ISO Brightness 81.3 82.6 82.6 82.7 (%)

It was found that similar residual peroxide and ISO brightness results were obtained even when using reduced amounts of hydrogen peroxide when the magnesium hydroxide was added first.

It should be noted that the present invention may comprise any combination of the features and/or limitations referred to herein, except for combinations of such features which are mutually exclusive. The foregoing description is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims. 

1. A method comprising: mixing an alkaline earth metal oxide or an alkaline earth metal hydroxide with a component selected from the group consisting of: a mineral acid, sulphur dioxide, carbon dioxide, an alkaline earth metal sulfate, an alkaline earth metal carbonate, another an alkaline earth metal salt and combinations thereof.
 2. A method according to claim 1, wherein the mixing step is performed within an aqueous pulp bleaching composition.
 3. A method according to claim 2, wherein the mixing step is performed by of adding sulphuric acid, sulphur dioxide, or carbon dioxide to a paper bleaching pulp comprising the alkaline earth metal oxide or alkaline earth metal hydroxide.
 4. A method according to claim 1 comprising forming a composition that comprises (i) the one or more alkaline earth metal oxides or alkaline earth metal hydroxides; (ii) the one or more alkaline earth metal salts; and (iii) one or more pitch control additives.
 5. A method according to claim 1, wherein the mixing step takes place in the presence of a said control additive is selected from talc, micronized talc, bimodal talc, cationic talc, bentonite, zeolite, diatomaceous earth, cationic mica, hydrophobic carbonates, resin-decomposing enzymes, cationic polymers designed to capture and immobilise resins or resinous components on fibre surfaces, aluminium sulphate, polyaluminium chloride, and dispersing agents specifically designed for the dispersion of resins and resinous components, and mixtures thereof.
 6. A method according to claim 1, wherein the said alkaline earth metal is selected from magnesium, calcium, and mixtures thereof.
 7. A composition comprising (i) one or more alkaline earth metal oxides or alkaline earth metal hydroxides; and (ii) one or more alkaline earth metal salts.
 8. A composition according to claim 7, further comprising a pitch control additive is selected from talc, micronized talc, bimodal talc, cationic talc, bentonite, zeolite, diatomaceous earth, cationic mica, hydrophobic carbonates, resin-decomposing enzymes, cationic polymers designed to capture and immobilise resins or resinous components on fibre surfaces, aluminium sulphate, polyaluminium chloride, and dispersing agents specifically designed for the dispersion of resins and resinous components, and mixtures thereof.
 9. A composition according to claim 7, wherein the alkaline earth metal is selected from magnesium, calcium, and mixtures thereof.
 10. A composition according to claim 7, wherein the composition is an aqueous slurry.
 11. A composition according to claim 10, wherein said aqueous slurry has a total solids content of from 10 to 75 wt. %, on the basis of the total weight of the slurry.
 12. A composition according to claim 7, wherein the weight ratio of (i) the one or more alkaline earth metal oxides or alkaline earth metal hydroxides and (ii) the one or more alkaline earth metal salts is from 1:10 to 100:1.
 13. A composition according to claim 10, further comprising at least one of the following additives: (a) dispersants; (b) fillers; (c) surfactants; (d) bleaching agents; (e) chelating agents; and (f) pH-buffering agents.
 14. A kit of ingredients comprising (i) one or more alkaline earth metal oxides or alkaline earth metal hydroxides stored in a first container, and (ii) either one or both of an aqueous mineral acid solution stored in a second container and a sulphur dioxide and/or carbon dioxide stored in a third container.
 15. The method of claim 1, further comprising bleaching or delignification of a pulp.
 16. The method of claim 15, further comprising reducing pitch in the pulp.
 17. The method of claim 15, further comprising protecting the pulp from depolymerisation and/or degradation.
 18. The method of claim 15, further comprising an acidic bleaching step.
 19. The method of claim 1, further comprising adding the product of the mixing step to a pulp and bleaching the pulp, wherein the product of the mixing step is added to the pulp prior to any addition of other alkaline bleaching agents, such as water glass, sodium silicate, NaOH, and/or hydrogen peroxide. 20-25. (canceled) 